Try with:
.activity-item:not(.activityinline) { border: none; padding: 0.1rem; }
It works more or less for me in Moodle 4.3.
The text and media area I haven't been able to adjust it.
I used simple lines <br> in a text and media area (old labels) to clarify and separate some content in 3.11 and now in 4.3 the line occupies 500% of the vertical space it occupied before. A nonsense.